A manual workflow is defined as a workflow that has no trigger and is instead manually started by a store manager. Existing workflows would be considered 'automatic' workflows in comparison.
A manual workflow would need a primary data type (e.g. order, customer, subscription) which would define which rules, actions and variables could be used in the workflow.
A manual workflow could be started from a related admin page. E.g. an order manual workflow could be initiated from the edit order page. Alternatively, manual workflows could be started from a custom interface that allows selecting a set of items to be triggered. It would be useful to select items by a date range and select multiple items by ID.
This system would be a replacement for the manual order and subscription trigger tools.
If you have any use cases for manual workflows please add them as a comment :)